Crystalline zeolite beta and its conventional preparation are described in U.S. Pat. No. 3,308,069 and Re 28,341; the entire disclosure of each is incorporated in this specification by reference. It has a distinctive X-ray diffraction pattern which identifies it from other known crystalline silicates.
U.S. Pat. No. 4,284,529 claims a catalyst composition comprising an inorganic matrix and the alkali metal form of a steamed crystalline zeolite having a silica/alumina mole ratio above 12, a constraint index of from 1 to 12 and an alpha value of not less than 5, the catalyst composition having substantially no activity for cracking n-hexane. Zeolite beta prepared with tetraethylammonium ions as reaction mixture directing agent as in U.S. Pat. No. 3,308,069 is used as catalyst for hydrocracking C.sub.5.sup.+ naphtha in U.S. Pat. No. 3,923,641.
A zeolite having the structure of zeolite beta is described in European Patent Application 55,046 (named "NU-2") and British Patent 2,024,790 (named "Boralite B"). Zeolite NU-2 is described in EU 55046 as being a member of the zeolite beta family of zeolites but with the distinction, among others, that it can only be prepared in pure form with products having a limited range of silica:alumina ratios, from 20:1 to 50:1. This distinctive character of NU-2 is affirmed by EU 187552. Zeolite beta, by contrast, may be prepared with silica:alumina ratios as high as 200:1 by the method disclosed by Wadlinger although higher values may be attained by dealuminisation, as described in U.S. Pat. No. 4,419,220.
When the proportion of silica in the zeolite is to be increased, resort may be made to dealuminisation methods but clearly, it would be desirable to produce zeolites directly with the desired silica:alumina ratio so as to avoid the subsequent dealuminisation step. However, the synthesis method may impose limits upon the silica:alumina ratio of the as-synthesised zeolite product. Zeolite beta, for example, is generally limited to a silica:alumina ratio of about 200:1.
Zeolites other than zeolite beta are shown prepared from various reaction mixtures having various silica/alumina reaction mixture mole ratios in U.S. Pat. Nos. 4,366,135; 4,297,335; 4,275,047 and 4,257,885. Preparation of zeolite ZSM-5 having a SiO.sub.2 content of more than 90% by weight is described in European Patent Application 40,444. European Patent Application 36,683 and British Patent 1,553,209 disclose reaction mixture useful for preparing zeolites other than zeolite beta.
U.S. Pat. Nos. 4,088,605 and 4,148,713 disclose crystalline particles of ZSM-5 structure having an aluminum-free outer shell.